DESCRIPTION: (Applicant's Abstract) Ex vivo T cell depletion (TCD), is[unreadable] highly effective for lethal graft-vs.-host disease (GVHD) prevention.[unreadable] However, T cell removal may compromise beneficial donor T cell responses.[unreadable] Pharmacological agents are incompletely efficacious in preventing GVHD and[unreadable] have significant side-effects. Rather than broadly eliminate T cells or T[unreadable] cell subsets, the applicants propose to functionally alter ex vivo the small[unreadable] proportion of donor T cells with anti-host alloreactive potential. In aim[unreadable] 1, two types of ex vivo approaches are proposed to inhibit CD4+ T cell[unreadable] mediated GVHD lethality. The first involves physically blocking the[unreadable] interaction of cell surface determinants on donor T cells with those on host[unreadable] antigen-presenting cells. The CD40L, CD40 and CD28/B7 pathways will be[unreadable] targeted. The second involves targeting intracellular signalling pathways[unreadable] (PI3K, Jak3) required for optimal T cell responses. A series of studies[unreadable] using wild-type and various knockout (ko) mice are proposed in aim 1 which[unreadable] will investigate the mechanism(s) responsible for inducing alloantigen[unreadable] hyporesponsiveness. In aim 2A, successful tolerization approaches will be[unreadable] applied to CD4+ and CD8+ T cell containing donor inoculum. In aim 2B, T[unreadable] cell receptor (TCR) transgenic systems will be used to track CD4+ or CD8+[unreadable] TCR clonotypes that are capable of specifically responding to alloantigen or[unreadable] nominal antigen. These studies will provide important information regarding[unreadable] the mechanism(s) responsible for tolerance induction and the specificity of[unreadable] these approaches as measured by bystander effects on nominal antigen[unreadable] responses. In aim 2C, the effects of tolerance induction on[unreadable] graft-versus-leukemia (GVL) will be analyzed using a delayed lymphocyte[unreadable] infusion model system. This application will provide new strategies for ex[unreadable] vivo GVHD prevention and will provide important data on the mechanism(s) and[unreadable] specificity of such approaches in clinically relevant model systems.[unreadable]